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THE effect of three different fan control strategies on gas-fired unit heater performance was investigated by modifying unit heaters in two identically equipped greenhouses so that they could be operated in one of three modes: (a) with the fan controlled by a thermal switch so that the fan ran only when the flue gas temperature was above 38 C, (b) with the fan running continuously, and (c) with the fan controlled in standard mode, i.e. so that the fan ran only when the burner was on. The heater in House 1 was alternated between control modes (a) and (b) while the heater in House 2 was operated in standard control mode, then the heater in House 2 was alternated between modes (a) and (b) while the heater in House 1 was operated in the standard mode. No significant difference was found in the savings produced using mode (a) versus that produced using mode (b). The total fuel savings for the testing period was 11.6%. When the daily fuel savings were normalized for differences in temperature between houses the savings increased to 16.9%. Both actual and normalized fuel savings were found to be negatively correlated with the amount of heater activity. The thermal switch mode was found to offer a low cost alternative for increasing unit heater efficiency.